Casto, a native of Salem, Ore., hit his first major league homer with his father and various other family members in attendance on Father's Day at Safeco Field.
Washington's victory extended Seattle's home losing streak to seven straight games.
Catcher Jesus Flores knocked in two runs. Lastings Milledge had three hits and one RBI, while three players had two hits for the Nationals, who lost two of three at home to Pittsburgh before going to Seattle.
The winning pitcher was reliever Jesus Colome (2-1).
Seattle's Ichiro Suzuki had two hits and one RBI. A run scored on Elijah Dukes' third-inning fielding error, but no other player had more than one hit for the Mariners, who have lost 10 of their last 13 games overall.
The losing pitcher was reliever Mark Lowe (1-3).